1. Field of the Invention
One disclosed aspect of the embodiments relates to an image forming apparatus that includes a plurality of light emitting elements arrayed so as to expose different positions in a longitudinal direction of a photosensitive member.
2. Description of the Related Art
An electrophotographic image forming apparatus that uses a light-emitting diode (LED) as a light source to expose a photosensitive member has been known. Such an image forming apparatus includes an exposure head that includes a plurality of LEDs arrayed in a longitudinal direction of the photosensitive member, so that one pixel is formed by each LED. The number of arrayed light emitting elements is determined based on the width and the resolution of the image forming area in the longitudinal direction of the photosensitive member. For example, for a 1,200 dpi printer, since the interval between pixels is about 21.1 μm (truncated to one decimal place), the light emitting elements are arranged so that the interval between adjacent light emitting elements forming pixels is 21.1 μm. Since such an LED exposure type image forming apparatus does not require a polygon mirror, reducing the size of the apparatus and reducing costs is easier, compared with a light beam scanning type image forming apparatus that uses a polygon mirror.
On the other hand, an LED exposure system suffers from the problem that when there is unevenness in the light amount of the respective LEDs, image defects such as streaks are produced in the rotation direction of the photosensitive member. To suppress such unevenness in the light amount of the LEDs, light amount adjustment is performed for each LED before shipment.
Further, LEDs also suffer from light amount deterioration, in which the amount of emitted light deteriorates over time. Consequently, if the usage frequency (cumulative number of times of light emission and cumulative duration of light emission) is different among the LEDs, a difference occurs in the level of deterioration among the LEDs, which results in unevenness in the amount of emitted light. For example, when the user prints a large quantity of the same image for a long period, or when a formulaic image of a line (ruled line etc.) along the rotation direction of the photosensitive member is included in the print image, a difference in the usage frequency of the LEDs occurs based on the output image, which results in a difference in the amount of emitted light of the LEDs.
To suppress the unevenness in the amount of emitted light that occurs due to differences in the usage frequency of each LED, Japanese Patent Application Laid-Open No. 2007-62020 discusses an image forming apparatus that stores data relating to the usage frequency of each LED (cumulative number of times of light emission or cumulative duration of light emission), and controls the light amount of each LED based on that data.
However, in Japanese Patent Application Laid-Open No. 2007-62020, a memory for storing the data relating to the usage frequency of the LEDs needs to be provided for all the LEDs. Specifically, Japanese Patent Application Laid-Open No. 2007-62020 suffers from the drawback that a large-capacity memory has to be provided.